Return error messages from Return and Satisfy

gigaparsec.go

@@ -18,16 +18,18 @@ type Result[In, Out any] struct {
 
 type Message struct {
 	Pos      uint64
-	Msg      string
+	Got      string
 	Expected []string
 }
 
-type ParseError struct {
-	Message
-}
+func MessageOK(pos uint64) Message { return Message{Pos: pos} }
+
+func MessageEnd(pos uint64) Message { return Message{Pos: pos, Got: "end of input"} }
+
+type ParseError Message
 
 func (pe ParseError) Error() string {
-	return fmt.Sprintf("parse error: %d: %s", pe.Message.Pos, pe.Message.Msg)
+	return fmt.Sprintf("parse error: %d: %s", pe.Pos, pe.Got)
 }
 
 type State[T any] cursor.Cursor[T]
@@ -36,24 +38,35 @@ type Parser[In, Out any] func(State[In]) (consumed bool, reply Result[In, Out],
 
 func Return[In, Out any](value Out) Parser[In, Out] {
 	return func(state State[In]) (bool, Result[In, Out], error) {
-		return false, Result[In, Out]{Value: value, State: state}, nil
+		return false, Result[In, Out]{
+			Value:   value,
+			State:   state,
+			Message: MessageOK(state.Pos()),
+		}, nil
 	}
 }
 
 func Satisfy[T any](pred func(T) bool) Parser[T, T] {
 	return func(state State[T]) (bool, Result[T, T], error) {
-		b := make([]T, 1)
-		n, next, err := state.Read(b)
+		token := make([]T, 1)
+		n, next, err := state.Read(token)
 		if errors.Is(err, io.EOF) {
-			return false, Result[T, T]{}, ErrNoParse
+			return false, Result[T, T]{}, ParseError(MessageEnd(state.Pos()))
 		}
 		if n != 1 {
 			panic(fmt.Sprintf("expected 1 element from Read, but got %d", n))
 		}
-		if pred(b[0]) {
-			return true, Result[T, T]{Value: b[0], State: next}, nil
+		if pred(token[0]) {
+			return true, Result[T, T]{
+				Value:   token[0],
+				State:   next,
+				Message: MessageOK(state.Pos()),
+			}, nil
+		}
+		return false, Result[T, T]{}, ParseError{
+			Pos: state.Pos(),
+			Got: fmt.Sprint(token),
 		}
-		return false, Result[T, T]{}, ErrNoParse
 	}
 }
 
@@ -68,6 +81,7 @@ func Bind[In, A, B any](p Parser[In, A], f func(A) Parser[In, B]) Parser[In, B]
 	}
 }
 
+// TODO Add error propagation
 func Choose[In, Out any](p, q Parser[In, Out]) Parser[In, Out] {
 	return func(input State[In]) (bool, Result[In, Out], error) {
 		consumedP, replyP, errP := p(input)